A convenient and popular means for storing and using magnetic recording tape is the two-reel cassette, in which a length of magnetic tape is stored on a supply reel, with the free end of the roll of stored tape threaded over a tape path defined by a series of tape guides, to a takeup reel. The ends of the recording tape are typically attached to the reels through lengths of film, called leaders, having a width similar to that of the tape. One end of each leader is attached to an end of the magnetic tape and the free end of each leader is secured to the hub of a respective reel to enable the magnetic tape to be wound onto the takeup reel while being unwound from the supply reel.
The leader is clamped to the hub in a manner to sufficiently robust to enable the leader to withstand the relatively high tensile forces which are likely to be applied to it in use, without detaching from the reel hub.
The supply reel, takeup reel, and the tape guides are contained in a cassette housing, which can be inserted into a playing or recording apparatus (VCR) for recording or playback of the tape. Examples of magnetic tape cassettes are VHS and 8 mm video cassettes, which are popular for home use, D2, D3, and Betacam cassettes, which are often used in professional studio work.
Videocassette reels typically include two opposing flanges connected by a hub. Usually, the lower flange and the hub are molded as a single piece and the upper flange is molded separately and is attached to the hub before the hub is inserted into the cassette housing and before any tape is wound on or connected to the reel. These configurations apply to many different format videocassettes. Some examples of tape reels are shown in Iwahashi U.S. Pat. Nos. 4,807,826, Nakagawa 4,403,749, and Nelson et al. 4,184,650.
Known tape reels have clamping devices for fastening the tape to the reel. As shown in FIG. 1, an opening 12 is formed in the hub 10 of the tape reel. One leader is placed into the opening 12 and a tape locking piece, such as a clamp 14, is pushed into the opening 12 to fasten the tape to the hub 10.
The bottom of the hub 10 is open to create a receptacle for receiving and engaging a VCR drive spindle. A top view of the hub 10 reveals an outer cylinder 18, which forms the tape winding surface 20, and an inner structure such as an inner cylinder 22. The inner cylinder 22 is closed and forms the drive spindle receptacle. The top surface 24 of the inner cylinder 22 can serve as at least part of a connection point for the upper flange. A plurality of ribs 26 and posts 28 can be formed inside of the outer cylinder 18 for strength and to improve moldability. The inner cylinder 22 can be connected to the outer cylinder 18 via some of the ribs 26.
A feature which contributes to high levels of cassette noise is the use of pressure buttons which pass through the top cassette wall to apply a downward axial force to the centers of the reels. Pressure buttons of this type are disclosed in U.S. Pat. No. 5,181,153, which shows cassettes of both the Betacam and digital video type.
Means for reducing cassette noise from various sources are known. Reduction of noise at high tape transport rates caused by tape guide rollers are disclosed in published German Patent Application No. DE 4 004 675. In this reference, two guide rollers, called deflection rollers, have internal bores having a convex shape, and the ends of the bores provide projections beyond the end faces of the rollers. Published French Patent Application No. FR 2 488 029 discloses tape cassette reels in which the resin used in their production includes graphite, presumably to reduce friction, and hence, noise and wear.
The inventors have found that a significant portion of high speed transport noise generated by cassettes having sliding reel pressure buttons is caused by vibration resulting from unbalance of the reels.
Reels used in magnetic tape cassettes are inherently unbalanced. Due to the additional material used to form the structure surrounding the clamp-receiving opening, and due to the higher density material used for the clamp, the side of the hub that receives the clamp is heavier than the other side. The hub is unbalanced. This problem is inherent in the patented reels described above. In the Betacam Large SP format, for example, the center of mass of the hub is offset from the axis of rotation toward the clamp by approximately 0.025 cm (0.010 inch). Tape reels which do not use any clamping device to hold the end of a magnetic tape onto the hub can be inherently balanced, simply because of the rotational symmetry of the reel, but these reels do not provide the means for secure clamping which is needed in many magnetic tape cassette reels.
An example of a clamping device for a reel hub is disclosed in FIG. 1 of U.S. Pat. No. 4,807,826, in which the hub surface 11 has a gap 30 for inserting a clamp 31. U.S. Pat. No. 4,664,328 discloses a hub configuration in which the hub has a relatively thin cylindrical wall. A structure 13 provides adequate strength to the tape clamping gap in this hub, and the mass of the structure 13 contributes to the overall unbalance of the reel.
In addition to the unbalance contributed by the structure surrounding the tape gap, the tape clamp itself can add unbalanced mass, due to its material requirements. A typical material for a cassette tape reel is ABS or injection molded high impact polystyrene (HIPS). This material has been found to be unsatisfactory for the tape clamp, however, due to its lack of stiffness and strength. It has been found that a much better material for the tape clamp is acetal, an example of which is Delrin, available from E.I. DuPont de Nemours and Company. The specific gravity of a typical grade of Delrin is about 1.42, while the specific gravity of a typical HIPS is about 1.03. A typical tape clamp for a hub for 0.5 inch wide tape for use in a Betacam cassette weighs about 0.25 grams, which is 0.07 grams more than it would weigh if made from HIPS. This added weight at a localized area on one side of the hub further contributes to unbalance of the overall reel.
During fast winding, such as in the fast forward and rewind modes of operation and during factory winding, the weight and mass imbalance causes the hub to rotate with an eccentric motion around the axis of rotation. This causes vibration and uneven winding of the tape on the hub, and can lead to edge damage on the tape. Also, in cassette configurations that use reel pressure buttons to apply a pressure to the top center of the reel, the wobble vibration of the reel vibrates the pressure button. These vibrations, which are also transferred to the cover and base of the cassette, create undesirable noise as the cassette functions in a VCR. Because the walls of cassettes are relatively thin compared to the length and width of the cassette, cassette walls tend to act as sounding boards to produce significant noise levels from even low levels of vibration.
No known cassette reels address this balance problem. None of the art suggests that balancing of magnetic tape cassette reels would be useful in reducing acoustic noise generated by cassettes, nor does any of the art suggest a way to produce mechanically balanced magnetic tape reels by injection molds, while retaining satisfactory dimensional tolerances and reel quality.